1. Technical Field of the Invention
The present invention relates to satellite communication systems and, in particular, to utilizing key transforms to discriminate between beams in a multiple-beam satellite communication system.
2. Description of Related Art and Objects of the Invention
Mobile wireless communication is becoming increasingly important for safety, convenience, and efficiency. One prominent mobile communication option is cellular communication. Cellular phones, for instance, can be found in cars, briefcases, purses, and even pockets. Cellular phones, like most mobile communication options, rely on the transmission of electromagnetic radiation from one point to another.
In general, a cellular mobile communications system is composed of many cells, each with a base station antenna for receiving transmissions. From the base station, the cellular system has interfaces for routing a call through or to the land-based, or terrestrial, telephone network, often referred to as the public switched telephone network (PSTN). The base stations form one half of the cellular system. Cell phones, called mobile stations, mobile terminals, or merely terminals, form the second half of the cellular system. In short then, electromagnetic radiation transmissions between terminals and base stations are an essential component of cellular systems, and such transmissions must be optimized by the cellular system to maximize cellular phone service, quality, and security.
Security becomes even more difficult to ensure when using satellites in a mobile communications system because the electromagnetic beams transmitted from the satellites can overlap countries and even continents. Therefore, a person on one continent can eavesdrop on another person""s conversation on an entirely different continent. Nevertheless, satellite-based communication systems are desirable because they enable global coverage without necessitating closely-spaced cellular base stations.
Throughout the following, the terms cellular mobile telephone, cellular phone, cellular telephone, mobile telephone, phone, radiotelephone terminal, cellular terminal, mobile terminal, and xe2x80x9cterminalxe2x80x9d may be used equivalently to refer to a wireless communications device capable of wirelessly transmitting and receiving data. Also, the terms radiotelephone network, cellular system, and cellular network are used equivalently to refer to a wireless communications system which provides wireless data connections between two or more terminals or between two or more terminals and other equipment.
Cellular mobile telephones and other such radio communications devices are usually designed to meet the requirements of a limited number of standards in common use in the world. Many countries choose the same standard; for example the pan-European Global System for Mobile Communications (GSM) is used by 14 European countries, Australia, and some Middle Eastern countries. The U.S. digital cellular standard, IS-54B Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard (available from the Telecommunications Industry Association, 2001 Pennsylvania Avenue, N.W., Washington D.C., 20006) (IS-54B) is used, for example, in the USA, Mexico, Canada, and South America. As a result of their being only a handful of mobile communication system standards, as well as economies of scale, mobile telephones are produced in enormous volume and generally adapted so that the design is the same for many markets.
The current invention belongs to the field of anti-fraud and privacy systems for public or private wireless communications systems, such as digital cellular telephone systems. Such systems may use authentication procedures for verifying the identity of a mobile phone attempting to access the network or to verify the network to the phone, as well as scrambling to prevent traffic broadcast to one mobile accidentally or deliberately being received by another.
A problem that has arisen before is that mobile telephone numbers are re-used in other countries and therefore not guaranteed to be unique in the whole world. Thus, an IS-54B telephone from a Far Eastern country brought to the United States by a traveler may have the same telephone number (mobile identification number, or MIN) as a xe2x80x9cnativexe2x80x9d American phone, and can sometimes access the network in an unforeseen manner, including fraudulently. It is an object of anti-fraud systems to prevent such unauthorized access.
Anti-fraud systems make use of a secret number like a PIN code embedded into every phone. The number stored in the phone is also stored in the telephone exchange belonging to the operator with which the user has a subscription. A phone attempting to access a network is xe2x80x9cchallengedxe2x80x9d with a random number which it is invited to combine with the secret PIN code in a defined way and to return a result. The network meanwhile contacts the phone""s home exchange with the random number and invites it to do likewise. If the result from the phone matches that from the home exchange, the phone is admitted to the network, and the network is fairly certain that its bill for services can be sent to the phone""s home operator and that it will be honored.
While it may not be too difficult to ensure that unique PIN codes are issued to all phones registered with a particular operator, it is not so obvious how to coordinate PIN code issuance between operators to guarantee uniqueness, without compromising security by allowing too many organizations to have access to secret information. It is not very likely that all the coordination required could be achieved between continents. Therefore, the invention provides a means of discriminating the security information between different networks such that uniqueness of the information is not a necessity.
U.S. Pat. No. 5,091,942, granted to applicant, which is hereby incorporated by reference in its entirety herein, discloses a bilateral authentication procedure that verifies a mobile phone to the network as well as the network to the phone. The inventive bilateral authentication system also produces as a byproduct a temporary variable to be used for scrambling traffic. U.S. Pat. Number 5,060,266, also granted to applicant, which is also hereby incorporated by reference in its entirety herein, describes a type of scrambling system suitable for such purposes. Furthermore, a suitable algorithm is described in U.S. patent application Ser. No. 07/556,358 (Dent, filed Jul. 20, 1990), which is also hereby incorporated by reference in its entirety herein.
The referenced prior art discloses use of a 64-bit temporary key, which is produced during the authentication procedure, to generate with the aid of a speech frame or time division multiple access (TDMA) transmission frame counter, a block of keystream bits for every frame which may be exclusive-ORed to traffic data to prevent it being received by a radio not in possession of the same 64 bits. The known prior art does not provide a means to ensure that radios of the same design, delivered to perhaps different continents and accidentally in possession of the same 64-bit key, cannot receive or transmit the same signal.
It is an objective of the invention to provide a means whereby portable communications devices such as cellular phones can be delivered all over the world with the same design without compromising the security of anti-fraud and privacy features in any one country or continent. This is particularly desirable in connection with global satellite communications systems which may be constructed using multiple-beam satellite systems as described, for example, jointly by U.S. Pat. Nos. 5,594,941, 5,555,271, 5,619,503, 5,619,210, 5,594,776, 5,535,432, and 5,610,559, and by U.S. patent applications Ser. Nos. 08/225,399 (filed Apr. 8, 1994) and 08/368,877 (filed Jan. 5, 1995) which is a continuation-in-part of 08/179,958 (filed Jan. 11, 1994). All disclosures of which are hereby incorporated by reference in their entirety herein.
A particular case of the problems referred to above arises in a global satellite communications system that attempts to use a limited number of satellites to provide service to subscribers all over the globe. The satellite telephones in question can, in principle, be of hand or portable size and be of the same design for use all over the globe. The problem of ensuring key uniqueness with a global number of subscribers is more difficult than with a national number of subscribers. Complications arise also due to the satellite system being a single network with a subscription that is not linked to any particular nation. It may thus arise that telephones could be purchased in one country for use in another, yet it is desirable to ensure that anti-fraud steps used in one part of the world do not cause weaknesses in other parts of the world, as stated above.
In addressing the security issues in global satellite communication systems, it must be considered that a desired feature of such a system is that a subscriber shall be able to make and receive calls to his same telephone number wherever he temporarily happens to be. This requires subscribers to have a globally unique telephone number embedded into their satellite/cellular telephones. The method by which a particular telephone is reached is described in the aforementioned U.S. Patents and Patent Applications, which have been incorporated by reference herein in their entirety.
In distinction to cellular networks that more-or-less coincidentally decide to adopt one or another of the cellular standards used in other parts of the world, without the specific intention of serving visiting subscribers from those other places, a satellite mobile system specifically aims to serve subscribers that have roamed from one part of the globe temporarily to another. Prior art security and anti-fraud measures, however, have heretofore failed to address the needs of a satellite mobile system where subscribers are expected to transport their mobile telephones across national borders and even onto different continents.
The above deficiencies in the prior art are overcome by use of the invention of the parent application as adapted and extended by the further descriptions herein. The current invention has the following objects (and others not specifically listed):
An object of the invention is to provide a technique for discriminating the security information between different networks such that uniqueness of the information is not a necessity.
Another object of the invention is to provide a technique whereby portable communications devices such as cellular phones can be delivered all over the world with the same design without compromising the security of anti-fraud and privacy features in any one country or continent.
Yet another object of the invention is to provide a technique to authenticate a subscriber registered in one part of the world to receive service in another part of the world, which parts of the world are not even both simultaneously visible from the same satellite.
A still further object of the invention is to prevent unauthorized reception of traffic transmitted from a satellite to a subscriber in one part of the world by a receiver in another part of the world in which the same satellite is visible.
The present invention provides a method and apparatus in a mobile communication system. The mobile network provides service to mobile terminals via at least one orbiting satellite in communication with a network of ground stations. This mobile, satellite-based communication system can cross national borders as well as overlap continents.
The technique of the present invention enables ciphering and deciphering of signals transmitted between a foreign satellite gateway and the mobile terminal. A key variable is transformed and used for scrambling mobile data traffic between the mobile terminal and the foreign satellite gateway. The scrambling varies depending on the selected cipher mode, as can be transmitted by the foreign satellite gateway. Transformation is accomplished by passing portions of the key variable, or related or derivative variables, through an authentication algorithm and a series of S-boxes, which provide a mapping between inputs and outputs. This produces a session key (S-key) that dictates the scrambling code.
In one embodiment, a foreign satellite gateway initially determines the identity of a roaming terminal. The roaming terminal""s identity can then be used to determine a home satellite gateway from which a key variable and possibly other cipher values can be obtained. The foreign satellite gateway next requests one or more cipher variables from the home satellite gateway.
Enciphered communication can then be enabled between the foreign satellite gateway and the roaming terminal after the foreign gateway communicates with the terminal""s native gateway. The native gateway transmits one or more cipher variables in the communication. The variables are used by the foreign gateway in conjunction with the mobile terminal to (i) enable ciphering, (ii) prove to the mobile that the gateway is legitimate, and (iii) prove to the gateway that the mobile is legitimate and is likely to pay for the resulting charges, termed authentication.
As stated above, the cipher mode can vary. Moreover, the value determining how to cipher the data traffic can be based on numerous factors, including aspects of the satellite communication system. For example, it can be based on (a) the home location register (HLR) on which the mobile has a permanent subscription, (b) the visiting location register (VLR) in which the roaming mobile is presently registered, (c) the territory in which the other call party is located, (d) the territory in which the ground station delegated to handle the call is located, and/or (e) subscriber profile information retrieved from the HLR.